1. Field of the Invention
The present invention relates to an information reading and writing device, for reading and writing an optical signal, which comprises a reading or writing head of an optical disk device or optical magnetic disk device used as a storage device of a computer.
In accordance with the development of a high performance computer, it becomes necessary to incorporate a storage device of large capacity into the computer. As the storage device to be incorporated into the computer, attention is given to a hard disk device and an optical disk device. Especially, the latter is advantageous in that the disk medium can be easily attached to and detached from the computer, that is, the disk medium can be easily replaced and further the capacity of the disk medium is large and the device is compact and the weight is small. For the above reasons, the optical disk device has become important.
2. Description of the Related Art
It is necessary to further reduce the price of an optical disk device or optical magnetic disk device. As a component to reduce the price, there is a strong demand for reducing the price of an optical head portion. Therefore, it has been conventionally necessary to provide a system in which the number of parts of an optical head can be decreased and the size and weight of the optical head can be reduced and also the labor to adjust the optical head can be reduced. On the other hand, in a disk medium in which both reading and writing of data can be conducted, there is a strong demand for increasing the luminous energy of laser beams, and also there is a strong demand for increasing the quantity of light for servo detection or transmission of an information signal.
For the above reasons, reduction of the number of parts, reduction of labor for adjustment and reduction of the size and weight are strongly required by integrating a large number of optical elements such as a polarization separating element of a polarization beam splitter or Wollaston prism, a beam splitter and a mirror into one package.
FIG. 1 is a view showing a signal detection optical system used for a conventional optical head of an optical magnetic disk. A laser beam emitted from a semiconductor laser 1, which is a light source, is made to be parallel by a collimator lens 2. After a portion of the parallel beam has been transmitted through a beam formation prism 3, it is directed to an optical magnetic disk, which is a storage medium, by a polarization beam splitter 4 and condensed onto an optical magnetic disk 6 by an objective lens 5.
When written data exists on the optical magnetic disk 6, an angle of polarization of light is rotated by the Kerr effect. That is, when written data exists on the optical magnetic disk 6, a laser beam of P-polarization light component used for detection is rotated by an angle of .theta.k by the Kerr effect. Therefore, a small quantity of S-polarization light component is generated. Polarization light components (S-component and P-component) of the beam which has returned from the optical magnetic disk 6 are reflected on the polarization beam splitter 4. A beam of light, which have been reflected on the polarization beam splitter 4, are reflected on the polarization beam splitter 7. Then, the beam of light passes through a 1/2 wave plate 8 and condenser lens 9. After that, the beam of light is separated into an S-polarization light component and a P-polarization light component by the polarization light beam splitter 10.
The S-polarization light component and the P-polarization light component are respectively incident on the light detectors 11, 12. When a difference in the intensity of these two signals are found, it is possible to detect a signal.
On the other hand, polarization light components of the beam which has returned from the optical magnetic disk 6 is reflected on the polarization beam splitter 4. A portion of the beam of light, which has been reflected on the polarization beam splitter 4, is reflected on the polarization beam splitter 7 and transmitted through the condenser lens 13. Then, the beam of light is incident on the beam splitter 14. After that, a focus is detected by the push-pull method in which the knife edge 15 and the optical detector 16 are used, and the track is detected by an optical detector 17.
As described above, in a conventional signal detection optical system of the optical head for the optical magnetic disk shown in FIG. 1, it is necessary to provide a large number of optical elements such as a polarization separating element of a polarization beam splitter or others. When these optical elements are incorporated, a large amount of labor is required for adjusting these optical elements.
There is proposed a technique in which the number of optical elements such as a beam splitter and others can be reduced by using a hologram element having a plurality of hologram regions in an optical head for an optical magnetic disk.
For example, Japanese Unexamined Patent Publication No. 3-150744 discloses a technique in which a hologram element which has two hologram regions arranged symmetrically with respect to a dividing line perpendicular to the track direction of a storage medium is used and a recording signal is detected by a difference between beams of diffracted light sent from these two hologram regions.
As described above, in the conventional optical head of the optical magnetic disk device, it is necessary to provide a large number of optical elements, and also a large amount of time is required for adjusting the optical elements.